The present invention relates in general to a broadband solid state radio frequency (RF) switch circuit for electrically coupling a common signal path, such as a single antenna, alternately to a transmitter and to a receiver. RF isolation switches using single pole double throw PIN diode switches provide a convenient way for coupling a common signal path to one of a plurality of circuit paths. PIN diode switches being completely electronic are superior in speed and reliability to mechanical relays for such a purpose. Furthermore, it is known in the prior art to use quarter wave length sections to provide signal isolation between the two circuit paths serviced by the same common signal path.
Radio transceivers require that a proper impedance be presented to the transmitter. Prior art circuits utilizing PIN diodes in combination with quarter wavelength transmission lines have been unable to provide this proper impedance over a significant bandwidth, for example 40 to 50 megahertz.
Numerous other circuits are known in the prior art in which the PIN diodes are forward and reversed biased to effect switching of a signal path from a single antenna to a transmitter or receiver. Circuits which use a series diode in the receive path require reverse biasing of the receive diode thus requiring the circuit to supply a significant DC back bias while transmitting. Especially for portable radio transceivers the typically maximum supply voltage is 12 volts or less and thus high voltages for providing diode bias are not available.
The present invention overcomes these draw backs in the prior art and provides a method for switching a single antenna between a receiver and a transmitter over a wide bandwidth. The invention provides broadband impedance matching not available in the present state of the art and uses solid state circuitry without high voltages for diode biasing.